1. Field of the Invention
The present invention relates to capacitors, and more particularly to capacitors for use in sensors.
2. Description of Related Art
A variety of sensors are known that utilize capacitors as sensor elements. One such example is in MEMS pressure sensors. Typical MEMS pressure sensors have a thin diaphragm formed in one of the substrates. The diaphragm forms a first capacitor plate, which is opposed to a second capacitor plate in an adjacent substrate. When pressure is applied to the diaphragm, the diaphragm with its capacitor plate bulges towards the stationary second capacitor plate. The deflection changes the spacing between the capacitor plates which causes a change in the capacitance, which can in turn be used to determine the pressure acting on the diaphragm.
There are certain limitations and disadvantages to the typical MEMS pressure sensors. Since the diaphragm is fixed around its edges, the edges remain relatively stationary, and only the center of the diaphragm undergoes the maximum deflection at any given pressure. Therefore, the change in capacitance occurs over a relatively small effective area of the capacitor plates, which limits the sensitivity of the sensor. The smaller the diaphragm size, the worse sensitivity becomes, since capacitance is directly proportional to capacitor plate area.
Another limitation on typical MEMS pressure sensors is that the movement of the capacitor plates is toward one another as pressure increases on the diaphragm. Capacitance is inversely proportional to the distance between the capacitor plates, so capacitors are more sensitive to changes in plate spacing when the plates are closer together. So in the traditional arrangement, at low pressures the plates are at their farthest, least sensitive spacing and at high pressures, the plates are at their closest, most sensitive spacing. As a result, traditional MEMS pressure sensors are particularly sensitive only over a limited range of pressures, and they are generally insensitive to small pressure fluctuations at lower pressures.
Yet another problem with typical MEMS pressure sensors is relatively high parasitic capacitance. Parasitic capacitance is any fixed capacitance that does not change with pressure, as opposed to active capacitance, which is the sensing element that changes with applied pressure. In MEMS pressure sensors where both the diaphragm substrate and opposed capacitor substrate are made of silicon, a relatively large parasitic capacitance is present across the insulating layer where the substrates are bonded together. This parasitic, constant capacitance dilutes the signal produced from the active capacitance of the sensing element that changes with pressure. In addition, traditional all silicon sensors suffer from breakdown of dielectric layers when the wafer stack is diced, which can cause shorting of the thin dielectric layers.
Parasitic capacitance can be addressed by making the substrate opposite the diaphragm out of a glass material, while the diaphragm substrate is made of silicon, which is metalized only above the diaphragm. While this reduces parasitic capacitance, the two dissimilar materials create thermal expansion-induced stress which varies by temperature. The capacitor and diaphragm are stress-sensitive, so the thermally induced stress has an undesirable effect on sensitivity and accuracy. Moreover, the glass material typically used has creep behavior which can cause drifts and shifts in the sensor.
Such conventional methods and systems have generally been considered satisfactory for their intended purpose. However, there is still a need in the art for pressure sensors that allow for improved performance. There also remains a general need in the art for capacitors that can address the types of problems described above for a range of applications. The present invention provides a solution for these problems.